The cutting insert used in milling cutter tools consists generally of a prismatic body having a planar base from which extend side surfaces--relief flank surfaces--which are intersected by the upper surface--the cutting rake surface. The cutting insert has at least one cutting edge which is defined as an edge created by the intersection of the cutting rake surface and the relief flank.
The insert is so held in the tool that the cutting edge is located in the circular cutting path of the tool and is inclined with reference to the rotary axis by an angle referred to as the axial rake angle.
The magnitude of the axial rake angle has a direct influence on the stability of the tool during operation and also on the durability of the cutting edge. In general, it is advantageous to have the inserts disposed at large axial rake angles. With tools having interchangeable inserts, providing for such large axial rake angles gives rise to a structural weakening of the tool. In consequence, the most common milling cutter tools are constructed for use with relatively low axial rake angles.
It is known that with inserts for milling cutter tools the relief angles employed are relatively large as compared with the angles used with stationary cutting tools, and this is particularly the case with milling tools having low diameter cutting paths. Increasing the relief angle leads to a weakening of the cutting edge and there is in consequence a limitation of the magnitude of the relief angle. The magnitude of the relief angle is determined in accordance with the material of the workpiece, the material from which the cutting edge is formed and the cutting path diameter of the tool. In general, with hard and tough workpiece materials, the relief angle is between 6.degree. to 8.degree. with tools of high speed steel, and between 5.degree. to 7.degree. with carbide tools. On the other hand, with workpieces of mild steels, cast iron, etc., the relief angle can be as high as 20.degree..
A rake surface of the insert is the surface against which the chips bear as they are being severed. Where the inclination of the rake surface is such as to make the cutting edge keener or more acute, the rake angle is defined as being positive. Where, however, the inclination of the rake surface is such as to make the cutting edge less keen and more blunt, then the rake angle is defined as being negative. It is well known that when the rake angle is increased in the positive direction, the required cutting force and the cutting temperature which is generated, will both tend to decrease. Furthermore, the tool life for such an insert, having a relatively high positive rake angle, tends to increase up to a certain optimum value of the rake angle. On the other hand, the size of the rake angle is limited by the properties of the insert material. Thus, it is not possible to increase the rake angle beyond a certain maximum value.
Thus, it is desirable, in order to achieve optimum cutting performance and tool life, that both the relief angle and the rake angle at all points along the length of the cutting edge be maintained as large as possible.
It has been found, however, that with peripheral rotary milling cutters, as a result of the location of the inserts so as to present an axial rake angle with respect to the longitudinal axis of the cutter, the relief angle presented by the cutting edge at its leading end is very much greater than the relief angle presented by the cutting edge at its opposite trailing end. On the assumption that the relatively smaller relief angle presented by the insert at its trailing end is, in fact, the minimum relief angle which should be employed, it will be realized that the relief angle at the leading end is unduly large, thereby leading to an increased danger of weakening of the cutting edge at this point and a consequent lowering of tool life. On the other hand, whilst the rake angle presented by the insert at its trailing edge is positive to the required degree, the rake angle presented by the insert at its leading end is unduly negative, thereby requiring high cutting forces to be applied to the cutting edge at this leading end, such high cutting forces leading to tool chatter, insert breakage and general shortening of tool life.
With known rotary milling cutters having interchangeable inserts located at an angle to the rotary axis of the tool, the inserts are formed with straight cutting edges. When such tools are used for milling a surface parallel to the rotary axis, the milled surface is not smooth, consisting of concave portions. This of course detracts from the quality of milling and results from the fact that the points of contact of the cutting edge with the workpiece are not all disposed at the same radial distance from the axis.